1. Field of the Invention
This invention is related in general to the field of inductors, in particular, to transformers, inductors, and coils which are mounted on printed circuit boards or the like.
2. Description of the Related Art
Foil windings are becoming usual in a variety of electrical inductance applications. They are used in coils, inductors, and transformers of many varieties and applications. Their uses range from children""s toys, to household appliances, to high technology and aerospace applications.
While the advantages of foil windings are acknowledged, there continues to be some problems associated with the use of foil windings and similar winding materials such as parallel bonded magnet wire. One problem is connecting the foil winding to round wires. Round wires are typically used to conduct electric current to or from the foil winding. The cross section of the round wire is usually significantly less than the cross section of the foil. The result is local heating at the connection point, loss of energy, and higher failure rate. Another problem is wrapping foil windings on a bobbin. Prior art bobbins (see FIG. 1) are useful when the windings comprise many layers of fine wire. However, as the width of the wire increases, the bobbin flanges cause the exits from early windings to interfere with later windings. This decreases the number of turns which the bobbin can accommodate on a single layer. This interference increases with wire size. The interference is even worse for parallel bonded magnet windings and even more serious for foil windings. For switching power supplies, increases in power levels and increases in switching frequency tend to require a decrease in the number of turns in a winding and an increase in the width of a winding turn. Under these conditions the interference caused by the exits from various windings becomes even more serious.
The prior art has attempted to solve these bobbin problems by cutting deep slots in the bobbin flanges to allow exits of early windings, however, increases in the width of the windings has required ever wider slots in the flanges.
Clearly there exists the need for an improved coil configuration which solves the problems associated with connecting a round wire to a foil winding, reduces interference from exits of windings, reduces energy loss, reduces local heating, and improves reliability.
The invention discloses a coil winding configuration for use in transformers, inductors, and the like. The invention is particularly useful with foil or parallel bonded magnet windings. End portions of a foil winding are cut into flag shapes and folded to form a stack of foil conductor. The stack configuration forms self leads of the foil winding and facilitates the winding""s exits from the coil. The self leads extend from the coil and are formed to reach to a printed circuit board (PCB). The self leads are strong enough serve as a mount for securing the coil to the PCB. The ends of the self leads are trimmed to fit through at least one hole in the PCB. After insertion, the layers of the self leads are bent in opposing directions to substantially block the hole, prevent extraction, and block the flow of solder through the hole. The self leads are then soldered to the board. A bobbin having discontinuous flanges facilitates the exits of the self leads from the coil.
The flag shaped pieces are formed by making longitudinal cuts in the ends of the foil conductor. One or more cuts are made depending on the desired shape of the resulting stack. The flag shaped pieces are folded to form a stack which is at an angle to the foil conductor. This process is performed for both ends of the foil conductor to form self leads for both exits from the coil. The dimensions of the stack are adaptable to many applications. After the foil is wound on the bobbin, the stacks extend from the coil and form the self leads.
The self leads preferably extend from opposing sides of the coil. The leads are bent as needed to reach a mounting board such as a PCB or the like. The leads are strong enough to function as stable mounts for the coil, transformer, or inductor. This eliminates that need for some other mounting fasteners thus reducing costs.
The leads are trimmed to fit through the receiving holes in the board. The preferred embodiment trims each lead into two legs which are inserted through two adjacent round holes in the board. After the leads are inserted into the holes, the layers are bent in opposing directions. This serves to secure the leads to the board and to block the holes to reduce the flow of solder through the holes during the flow solder process.
The new bobbin shape facilitates the exits of the self leads. A key feature are flanges of the bobbin which are discontinuous. Portions of the flanges are formed to be planar with the body of the bobbin. The preferred embodiment has four flange portions which are planar with the bobbin body. The concept may be adapted to various bobbin and core shapes. This new bobbin shape is especially useful with parallel bonded magnet wire or with foil windings. Using the new bobbin, conductor exits do not interfere with the windings. This typically allows for one more turn per layer of single or parallel bonded magnet wire than would fit on a traditional bobbin. The flanges still protect the winding""s insulation from the sharp corners of the core; but in their new position, they do not interfere with lead exits. Isolation between the primary and secondary windings are improved further by placing the primary and secondary lead exits on opposite sides of the core.
Therefore, an object of the invention is to provide an improved self lead winding configuration for coils, transformers, inductors, and the like.
A feature of the invention is a stacked self lead which serves as a mount to a board (e.g., PCB).
Another feature of the invention is a stacked self lead inserted through a hole in a board and blocking the hole by bending layers of the self lead in opposing directions.
Another feature of the invention is a bobbin having a discontinuous flange portion parallel with the body of the bobbin.
Advantages of the invention include reduced energy loss, eliminating local heating where a round lead previously connected to the foil winding, improved reliability, increased number of turns on a single layer of the bobbin (or reduced bobbin size), and isolation between primary and secondary windings.